43S W. Gross — Topaz and Garnet in Rhyolite. 



temporaneous or nearly so with the final consolidation of the 

 rock. The lithophysal cavities seem plainly caused by the ex- 

 pansive tendency of confined gases or vapors, while the shrink- 

 age cracks in the walls and white masses of the Nathrop rock 

 suggest the former presence of moisture. Certainly the history 

 of the lithophyses themselves embraces that of both topaz and 

 garnet. 



Composition of the rhyolites containing topaz. — Below are sub- 

 mitted quantitative analyses of the only rhyolites known to 

 contain topaz in the manner described. Under I is given the 

 analysis of the Chalk Mountain rhyolite or nevadite, by W. F. 

 Hillebrand ; under II that of the denser Nathrop rock, and 

 under III that of the Utah rhyolite, — both by L. G. Eakins. 

 Analysis III was made upon the small fragment above de- 

 scribed, which was sent me by Prof. Clayton, as typical of the 

 occurrence. 



I. II. in. 



Si0 2 74-45 69-89 '74-49 



A1 2 3 14-72 1794 14-51 



Fe 3 3 - none 0'39 0-57 



FeO ... 0.56 0-'52 0-32 



MuO 0-28* 0-23 trace 



CaO 0-83 trace 1-03 



MgO 0-37 0-14 trace 



K s O . 4-53 4-38 4-64 



Na 2 3-97 4-21 3-79 



Li 2 trace trace trace 



H 2 0-66 2-07 0-64 



P 2 5 0-01 trace 



100-38 99-77 99-99 



From the above it is seen that the three topaz-bearing rhyo- 

 lites agree quite remarkably in composition. They are all 

 silica-alumina-alkali rocks, other constituents being present in 

 almost insignificant quantities. It is however by no means a 

 rare composition which is possessed by these rocks. The ex- 

 amination of the rhyolites of the Great Basin, near the 40th 

 parallel, by the chemists of the King Survey showed several 

 rocks closely allied to these in composition and similar ones 

 have been described from other parts of the western United 

 States and from Europe. 



* As Mn0 2 in this rock. 



